This project will develop pressure plethysmometers for the determination o, body volume/density (and hence fat/lean ratio) for adult humans and for small laboratory animals. Pressure plethysmometry is the physical process of determining the volume of an object by measuring the differential pressure created when a known volume of air is injected into a chamber of known volume and comparisons are made of the pressure with and without the test object in the chamber. The volume of the test object is then determined by Boyle's law. The determination of density (weight/volume) allows weight to be partitioned into lean and fat compartments. Pressure plethysmometry appears to be extremely accurate as differential pressures can now be accurately and rapidly measured; and confounding lung and intestinal gas volumes, inherent sources of error in other technologies, are automatically corrected for. In addition, a plethysmometer is relatively inexpensive and can be operated without special training.